Surface Controlled Growth of Thin-Film Strontium Titanate Nanotube

19 Aug 2014 - School of Public Health, University of Washington, Seattle, Washington 98195, United States. § Qatar Energy and Environment Research ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/crystal

Surface Controlled Growth of Thin-Film Strontium Titanate Nanotube Arrays on Silicon Hoda Amani Hamedani,*,† Janin A Khaleel,‡ Klaus-Hermann Dahmen,†,§ and Hamid Garmestani† †

School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive N.W. Atlanta, Georgia 30332-0245, United States ‡ School of Public Health, University of Washington, Seattle, Washington 98195, United States § Qatar Energy and Environment Research Institute, Qatar Foundation, P.O. Box 5825, Doha, Qatar ABSTRACT: We demonstrate the vertical self-organized growth of thin-film SrTiO3 (STO) nanotube arrays (NTAs) on SiO2 substrate. The surface morphology and crystal orientation of the grains at the exterior wall of the backbone TiO2 nanotube arrays were found to play an important role in the growth rate as well as the final morphology of the STO NTAs. A formation mechanism is proposed that involves nucleation of SrTiO3 nanocubes through a semitopochemical route followed by a self-assembly process resulting from the Ostwald-ripening-assisted oriented attachment of SrTiO3 nanocubes. It was shown that under appropriate reaction kinetics the nanotube architecture of the overall template can be maintained to form STO NTAs. The application of this novel platform enables a controlled and efficient mass fabrication of STO NTAs on widely used inexpensive silicon substrates, which can potentially lead to full integration with electronics in the near future.

■

and photocatalytic hydrogen production16 and also in biomedical applications as a Sr delivery platform for osseointegration (which means the formation of a direct interface between an implant and bone, without intervening soft tissue) on Ti-based bone implants.17 The use of high surface area 1D materials can also offer the possibility of increasing the sensitivity and stability for various types of micro-electromechanical systems (MEMS) or nano-electromechanical systems (NEMS) based applications such as oxygen gas sensors. Despite recent progress in design and development of state-of-the-art nanostructured materials and the efforts toward fundamental scientific understanding of their intrinsic size-dependent properties, integration of such functional nanostructures into silicon wafers is indispensable for utilization of these nanostructures as building blocks of miniaturized microelectronic devices. Thus, development of thin-film SrTiO3 nanotube arrays on silicon wafer and investigation of their growth conditions can serve as a key step toward utilization of such materials in variety of applications. In this work, the fabrication and growth process of high quality thin-film SrTiO3 nanotube arrays (hereafter STO NTAs) on silicon wafers via electrochemical anodization and post-hydrothermal processes is described. To the authors’ knowledge, this is the first reported demonstration of the growth of thin-film STO NTAs on silicon substrates.

INTRODUCTION Complex metal oxides with perovskite structure, ABO3 (where A and B are cations), exhibit a wide range of functional properties, such as ferroelectricity, piezo- and pyroelectricity, and nonlinear dielectric and semiconducting behavior.1 Strontium titanate (SrTiO3) is an excellent and well-known example of this family. Interesting properties such as high temperature stability, a combination of electronic and ionic conductivity, and dielectric, photoelectric, and catalytic properties make this compound a great candidate for several different applications including as a sulfur tolerant anode of solid oxide fuel cells (SOFCs),2−4 in oxygen sensors,5−7 and in integrated optoelectronic applications.8 Recently much attention has been focused on synthesis, structural characterization, and physical properties of perovskite oxide nanotubes (PONTs). TiO2 nanotubes have been used as template for formation of several ternary metal oxide nanotubes such as barium titanate nanotubes (BaTiO3) via a hydrothermal reaction and sol−gel processing of TiO2 nanotubes in Ba(OH)2 aqueous solution.9 Other examples of PONTs including (Ba,Sr)TiO3 (BST) and Pb(Zr,Ti)O3 (PZT) have been fabricated using different methods.1,10 La0.8Sr0.2MnO3−δ/Zr0.92Y0.08O2 (LSM/YSZ) composite nanotubes are cosynthesized by a pore wetting technique as a cathode material for solid oxide fuel cells (SOFCs).11 In the past decade, many efforts have been devoted to the synthesis of SrTiO3 in the form of nanospheres,12,13 nanocubes,14 and star-shaped morphologies.15 Among different morphologies, one-dimensional (1D) SrTiO3 nanostructures have been shown to be very promising for use in photoelectrochemical applications such as dye-sensitized solar cells © XXXX American Chemical Society

Received: September 5, 2013 Revised: July 24, 2014

A

dx.doi.org/10.1021/cg500374m | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

Figure 1. (a) Cross-section FESEM image of the aqueous-derived as-anodized TiO2 NTAs on silicon at 10 V and anodization time of 30 min. (b) EDS spectra showing the chemical composition of the thin-film TiO2 NTAs and the substrate surface.

■

incidence X-ray diffraction (GIXRD) using an X’Pert PRO MRD diffractometer with Cu Kα radiation source. The surface properties and composition of the samples were analyzed by X-ray photoelectron spectroscopy using a Thermo Scientific K-Alpha XPS with an Al anode. A pole figure study of the crystal orientation was performed by X-ray pole-figure measurements using X’Pert PRO MRD X-ray diffractometer equipped with a pole-figure goniometer, operated at 40 kV and 45 mA and employing Ni filtered Cu Kα radiation. A polycapillary X-ray lens is used to generate a parallel beam followed by placing a 0.27° parallel plate collimator and 0.04 rad Soller slits in front of the detector.

MATERIALS AND METHODS

Titanium Thin-Film Deposition. The n-type (100) Si wafer (500 μm in thickness) was cut into 1 cm × 2 cm pieces and was used as substrate for thin-film deposition. Prior to deposition, the Si substrates were degreased by sonication in acetone, methanol, and isopropanol, rinsed with deionized (DI) water, and dried in a nitrogen stream, followed by oxygen plasma cleaning. The Ti film was then deposited onto Si substrate using dc magnetron sputtering. The chamber was first evacuated to the pressure of 2.0 × 10−7 Torr, and the argon gas pressure was then maintained at 6.0 × 10−4 Torr during the deposition. The sputtering power and the deposition temperature were held constant at 125 W and 500 °C, respectively. The sputtering rate of titanium was found to be ∼1 Å/s. Based on this rate, the sputtering time was set to obtain a maximum sputtered thickness of 6 μm for titanium. Growth of Thin-Film TiO2 Nanotube Arrays on Si Substrate. To evaluate the effect of starting TiO2 material on growth of STO NTAs, self-organized TiO2 nanotube arrays were grown by electrochemical anodization of Ti thin-films on Si substrates in two different electrolytes. In the first route, the TiO2 nanotube arrays were grown in a conventional two electrode cell in an aqueous solution of 0.5 wt % HF and 1 M H2SO4 and at a constant temperature of 4 °C. The anodization of Ti film in the aqueous solution was carried out at a constant voltage of 10 V for 30 min and 1 h. The second route was followed by room temperature anodization of the Ti thin-film in an organic electrolyte consisting of ethylene glycol (EG) and NH4F, at the constant voltage of 40 V and for the same durations. Both types of thin-film TiO2 NTA samples were then annealed in air at 420 °C for 4 h and the heating and cooling rate of 1 °C/s to form crystalline anatase phase. Growth of Thin-Film STO NTAs on Si Substrate. The two types of annealed thin-film TiO2 NTAs synthesized above were used in hydrothermal treatment for synthesis of thin-film STO NTAs on silicon. A 0.025 M strontium containing solution was prepared by fully dissolving 0.151 g of anhydrous Sr(OH)2 in 50 mL of DI water. The two types of annealed aqueous and organic-derived thin-film TiO2 NTAs on Si substrates (which were anodized for 1 h) were immersed in the solution in a 45 mL Teflon-lined autoclave and heated at different reaction temperatures up to 180 °C for different reaction times. After the hydrothermal treatment, the autoclave was subsequently turned off to cool to room temperature. The samples were ultrasonicated in DI water to remove the residues and dried in air for characterization. The pH of the starting solution was measured to be 12. Characterization. The morphology and composition of the thinfilmTiO2 NTAs and STO NTAs were examined using a field emission scanning electron microscope (FESEM-Zeiss SEM Ultra60) equipped with EDS (energy-dispersive X-ray spectroscopy) detector. The crystal structure of the samples was determined by transmission electron microscopy (Jeol-2011 TEM operated at 200 kV) and glancing

■

RESULTS AND DISCUSSION Formation of TiO2 NTAs on Si Substrate. Figure 1 shows the cross-section FESEM image and the EDS analysis of the asanodized thin-film TiO2 NTAs on the silicon substrate, which was obtained using aqueous electrolyte (hereafter named aqueous-derived). The first step of the anodization of Ti film begins with the oxidation of titanium and formation of a compact TiO2 layer controlled by the applied field. Then, the local field distribution correlated to the surface morphological fluctuations directs the dissolution of this compact oxide layer at specific sites on the surface due to the attack by F− ions. This results in formation of nanosized pores on the surface, which eventually grow vertically to form nanotubes in a steady-state condition.18 The EDS confirms the chemical composition of the film and the substrate. The formation of high quality nanotubes has been found to be achievable only below 5 °C in the HF-based aqueous electrolyte solution. This is in accordance with the findings of Macak et al. where the formation of nanotubes in similar HF-based electrolyte has been observed at temperature of 2 °C.19 Figure 2 shows the FESEM images of as-prepared aqueous and organic-derived TiO2 NTAs before and after hydrothermal treatment in Sr(OH)2 solution at T = 150 °C for 5.5 h. The outer diameter of both types of nanotubes is ∼50 nm, and the backside of the nanotubes in both types is closed. As can be seen in Figure 2a, the nanotubes grown in the aqueous solution are composed of stacked irregularly shaped rings. However, the organic-derived nanotubes shown in Figure 2b are composed of an ideal close-packed arrangement of regular-shape tubes composed of hexagonal plates that are stacked on top of each other. The SEM images of bottom sides of the nanotubes are shown in insets. The samples were prepared for bottom side imaging by clear cleavage of the nanotube membrane from the substrate. As opposed to the aqueous route, close-packed B

dx.doi.org/10.1021/cg500374m | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

arrangement of hexagonal-shape tubes morphology is generally a characteristic of the TiO2 NTAs synthesized in organic solutions.20 Figure 2c,d show FESEM images of the aqueous and organic-derived TiO2 NTAs after hydrothermal treatment in Sr(OH)2 solution. As can be seen, at the same reaction temperature and time, the nanotubes prepared by the aqueous route have remained intact and did not undergo the reaction, and their surface was instead covered with random precipitates (shown with arrows in Figure 2c). However, in the case of organic nanotubes, the initial stages of the nucleation process have occurred. As shown in the corresponding tables, the chemical composition of the film on the surface reveals that the Sr content is much higher in organic-derived TiO2 nanotubes than that of aqueous-derived nanotubes. Significant changes in the morphology after hydrothermal treatment in the organicderived nanotubes can be attributed to a large portion of Sr2+ ions being incorporated into the structure of these nanotubes as observed from the EDS results. This behavior can be associated with the difference in the defective state of the precursor (TiO2 NTs) surface, that is, the crystallinity of the exterior side of TiO2 nanotubes walls prepared in aqueous and organic solutions. For comparison of the morphology and the crystallinity of the exterior side of the TiO2 nanotubes walls, TEM images of a single nanotube from each type are taken (Figure 3). The fast Fourier transform (FFT) diffraction pattern taken from the exterior side of the aqueous-derived TiO2 nanotubes reveals the presence of an amorphous layer at the exterior side of the nanotube wall; on the other hand, in case of the organic-derived TiO2 nanotubes, the FFT diffraction pattern suggests that the exterior side of the nanotube wall is composed of crystalline grains with preferred orientation of the crystallographic (101) lattice planes extended to the outer layer of the nanotube wall. Such pattern reveals that a great portion of the grains in that selected area on the wall are aligned in a more or less identical crystallographic orientation. It was found that the arrangement of these planes with sharp edges at the outer circumference of the tube assists the formation of highly crystalline SrTiO3 crystals from organic-derived TiO2 NTAs compared with those formed from the aqueous-derived TiO2 nanotubes. This can be attributed to the difference between the surface properties of the two types of nanotubes in terms of the crystallinity, orientation of the grains, and grain morphology only at the exterior side of the nanotubes walls. In fact, the aqueous-derived nanotubes are composed of stacked rings that are covered with an amorphous layer around the nanotube. A

Figure 2. FESEM images of as-prepared aqueous (left column) and organic-derived (right column) TiO2 NTAs before hydrothermal annealing (a, b) and partially converted to STO NTAs after hydrothermal annealing (c, d) at T = 150 °C for 5.5 h. (e, f) EDS spectra taken from the areas shown in parts c and d, respectively, and the corresponding chemical composition of these samples shown in the table. The insets in parts a and b show the bottom side of the nanotubes.

Figure 3. (a) TEM and FESEM images (shown in insets) and the corresponding FFT diffraction pattern taken from the exterior side of the walls of single TiO2 nanotubes prepared in (a) aqueous and (b) organic solutions. C

dx.doi.org/10.1021/cg500374m | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

similar effect has been also reported where the morphology of TiO2 affects the growth of cubic SrTiO3.21,22 For instance, in the work of Kalyani et al., the hydrothermal crystallization of strontium titanate using single crystal anatase and hydrogen titanate (HT) nanowires as precursors resulted in the formation of primary nanocubes or nanoparticles, respectively, grown on the precursor surface.22 Despite both precursors being single crystal nanowires in their work, formation of SrTiO3 nanocubes is only observed from single crystal anatase driven by a topochemical reaction. When crystallographic incompatibility exists between precursor and the growing phase, as in the case of aqueous-derived TiO2 NTAs in this work, a topochemical transformation will not occur and the reaction will result in loose spherical particles or nonoriented polycrystalline aggregates as seen in Figure 2c. Such a difference in the crystallinity as well as the orientation of the grains at the exterior walls of the starting TiO2 NTAs was found to be effective in crystallization mechanism of SrTiO3 in terms of the growth rate and the final morphology of the nanotubes, which is discussed in detail later. Thus, the organic-derived TiO2 NTAs were used as the template and the titanium source for synthesis and investigation of the growth process of SrTiO3 NTAs. Figure 4 shows the FESEM image of the pure and hydrothermally treated TiO2 NTAs at various reaction times

Figure 5. GIXRD pattern of organic TiO2 NTAs converted to cubic SrTiO3 at T = 180 °C for 18 h and the quantitative phase analysis in the inset table.

between the SiO2 substrate and the SrTiO3 at this reaction temperature and time. The interplanar lattice spacings of 0.396 and 0.281 nm are calculated for SrTiO3 (100) and (110) planes, respectively. Figure 6 shows a low-magnification bright-field TEM image and the SAED pattern of the STO NTAs that were synthesized on silicon. The polycrystalline nature of the STO NTAs with rings indexed to (100), (110), (111), and (200) planes are observed. This reveals the formation of NTAs of cubic SrTiO3 after hydrothermal treatment of organic TiO2 NTAs at 180 °C for 18 h. Figure 7 shows the XPS core level photoemission spectra for O 1s, Ti 2p, and Sr 3d for synthesized SrTiO3 NTAs on the silicon substrate. Results from peak fitting on all spectra additionally confirmed the complete exchange of Sr2+ ions as well as formation of SrTiO3 with the elements in chemical states associated with SrTiO3. The shoulder peak on the higher binding energy side of the main O 1s peak at approximately 532.5 eV originates from the adsorption of hydroxyl groups on the surface.24 The Sr 3d, Ti 2p, and O 1s peak positions were found to be in agreement with the Ti−O and Sr−O binding energies in SrTiO3 after annealing confirming the formation of strontium titanate.25 Table 1 shows the EDS chemical composition of the SrTiO3 NTAs at two different areas on the surface and along the nanotubes shown in the FESEM images in the table. As seen in the FESEM images, the surface of the nanotubes is composed of nanocubes oriented around the nanotubes, and the walls of the nanotubes are composed of brick-like morphology oriented along the longitudinal direction. The EDS results show the chemical composition on the surface of the nanotubes, which is very close to stoichiometric ratio, that is, Sr/Ti = 1, whereas, the Sr/Ti is observed to be lower than the stoichiometric ratio as we go along the nanotubes toward the backside of the nanotubes where they attach to the substrate. This discrepancy can be attributed to the lower access of Sr2+ ions to the walls of the nanotubes compared with the surface, which is explained in more detail in the growth process section. Growth Process of Thin-Film STO NTAs on Si Substrate. Generally, two major parameters control the morphology and composition of the final product in the hydrothermal process. One of these parameters is the reaction temperature. It has been reported that the optimal reaction temperature of preparing cubic SrTiO3 perovskites is about 180 °C.21 This temperature also appears to be the optimum

Figure 4. (a) FESEM images of (a) organic-derived TiO2 NTs, (b) partially converted nanotubes to SrTiO3 at T = 150 °C after 5.5 h, (c) partially converted nanotubes to SrTiO3 at T = 180 °C after 5.5 h, (d) and SrTiO3 nanocubes at T = 180 °C after 18 h.

and temperatures. The corresponding GIXRD patterns of the SrTiO3 at T = 180 °C after 18 h is also shown in Figure 5. It can be inferred that the reaction temperature and more importantly the reaction time can significantly change the morphology of the final structure. Based on the results of quantitative analysis of GIXRD data, it was found that ∼84 wt % (∼79% volume fraction) of the TiO2 anatase has completely converted to cubic SrTiO3 according to SrTiO3 after 18 h at 180 °C (JCPSD card No. 89-4934). The average crystallite size of the SrTiO3 was found to be 23 nm by Scherrer analysis.23 Small peaks corresponding to TiO2 anatase with weight fraction of 16% were detected. A few additional weak peaks are observed in the XRD pattern (not indexed), which compose 2% results in formation of edge dislocations and vertical grain boundaries to relieve the high tensile strain, leading to films with mosaic structure or columnar grains rather than single crystal films.22 Since a = b in anatase, the mismatch between (100) (or (010)) anatase (d = 0.350 nm) and (100) SrTiO3 (d = 0.390 nm) is 3.2%. Thus, the observed morphology of the STO nanocubes with a small degree of crystallographic misalignment is directed by the need to release the tensile strain. Dissolution of TiO2 can only occur as long as its surface is in contact with the alkaline solution. When the cubic crystallites at the exterior side of the nanotube walls enter into the growth step, continuous nucleation inside the pore region inside the nanotubes (due to more availability of the reactants in this area) may be still taking place. Once the surface of the TiO2 nanotube is completely covered with SrTiO3 crystallites, the nucleation stops and the process is further accompanied by a change in the shape of the crystallites starting at the exterior walls of the nanotubes through a semitopochemical transformation to cubic crystals at the top surface and gradually along the walls of the nanotubes. Along with the growth and in order to further reduce the surface energy, the self-assembly of primary SrTiO3 nanocubes subsequently occurs. In this assembly process, the well-known oriented attachment of the SrTiO3 nanocubes is followed by Ostwald ripening to form the final STO NTAs architecture. As can be seen in HRTEM image of SrTiO3 nanocubes in Figure 8f and the schematic of Figure 8h, the adjacent SrTiO3 primary nanocubes tend to keep the same crystallographic orientation through their shared faces and on one side of the hexagon. However, due to template effects from the nanotube morphology and different nucleation rates at the interior side of the nanotubes versus the exterior side, the oriented attachment results in tilting of adjacent cubic blocks and formation of “L” shaped structures with concave interior and straight edges at the exterior as shown progressively in Figure 9A−D. The last step of such self-alignment takes place via the Ostwald ripening-assisted process in which the size of the crystals becomes large enough and comparable to the curvature of the nanotube. Due to steric constraints crystals start to selforganize in order to most effectively fill the available surface of the pore walls.36 While the small SrTiO3 nanocubes are unstable and easily dissolved, the relatively large nanocubes continue to grow because of the Ostwald-ripening mechanism. This process leads to the formation of large number of selfaligned nanocube domains (as shown in Figure 9E), which tend to grow to single crystal cuboids (Figure 9F). A similar selfassembly mechanism has been reported by Kuang and Yang for the formation of porous SrTiO3 nanocubes on protonated titanate nanosheet hierarchical spheres as precursors.26 In our work, the smallest crystallite size at the early stages of the growth was estimated as ∼20 nm from XRD data. However, after completion of the growth, the nanocubes were found to grow to a size as large as 100 nm. At this point when the size of

■

CONCLUSION The fabrication, characterization, and growth process of STO NTAs on silicon was investigated. Using a simple hydrothermal method, the growth morphology of STO NTAs on previously fabricated TiO2 NTAs on silicon was studied at various reaction times and temperatures. It was assumed that, during the formation of STO NTAs, the nucleation starts from the exterior side of the nanotube walls where the orientation of the crystals in the starting TiO2 NTAs plays an important role in the growth rate as well as the final morphology of the STO NTAs. It is observed that the STO NTA formation from the organicderived TiO2 NTAs occurs via a topochemical-mediated transformation where there is no crystallographic incompatibility between precursor and product; this is opposed to the case of aqueous-derived TiO2 NTAs due to presence of the amorphous layer on the nanotubes walls. The as-synthesized STO NTAs were found to be crystalline with ∼84 wt % transformation of organic-derived TiO2 to stoichiometric SrTiO3 at the appropriate reaction temperature and time of 180 °C and 18 h, respectively. Orientation studies and HRTEM analyses of the starting TiO2 and final STO NTAs revealed the formation of STO NTA nuclei primarily on the most available (001) family lattice planes of organic-derived TiO2 NTAs, the exterior side of the nanotube walls, which resembles the heteroepitaxial growth of SrTiO3 on other single crystalline substrates. Further, the self-assembly of SrTiO3 nanocubes is realized as a result of the Ostwald-ripening-assisted oriented attachment, which under appropriate reaction kinetics can maintain the nanotube architecture of the overall template to form STO NTAs. There is a critical point at which the nanocube size becomes comparable to the size of the pores and as a result the nanotubular structure starts to alter into a randomly porous structure.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was funded by Pacific Northwest National Laboratory (PNNL). The TEM work was carried out at Florida State University, and the TEM facility at FSU is funded and supported by the Florida State University Research Foundation, National High Magnetic Field Laboratory (NSFDMR-0654118), and the State of Florida.

■

REFERENCES

(1) Zhu, X.; Liu, Z.; Ming, N. J. Mater. Chem. 2010, 20, 4015−4030. (2) Blennow, P.; Hansen, K. K.; Reine Wallenberg, L.; Mogensen, M. Electrochim. Acta 2006, 52, 1651−1661. (3) Savaniu, C. D.; Irvine, J. T. S. Solid State Ionics 2011, 192, 491− 493. (4) Kurokawa, H.; Yang, L.; Jacobson, C. P.; De Jonghe, L. C.; Visco, S. J. J. Power Sources 2007, 164, 510−518. (5) Hara, T.; Ishiguro, T.; Wakiya, N.; Shinozaki, K. Jpn. J. Appl. Phys. 2008, 47, 7486−7489. H

dx.doi.org/10.1021/cg500374m | Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

(6) Hara, T.; Ishiguro, T. Jpn. J. Appl. Phys. 2009, 48. (7) Voigts, F.; Damjanovic, T.; Borchardt, G.; Argirusis, C.; MausFriedrichs, W. J. Nanomater. 2006, No. 63154. (8) Lee, D. K.; Cho, I.-S.; Yim, D. K.; Noh, J.-H.; Hong, K. S.; Kim, D.-W. J. Ceram. Soc. Jpn. 2010, 118, 876−880. (9) Chen, Y.-Y.; Yu, B.-Y.; Wang, J.-H.; Cochran, R. E.; Shyue, J.-J. Inorg. Chem. 2008, 48, 681−686. (10) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446−2461. (11) Zhang, N.; Li, J.; He, Z.; Sun, K. Electrochem. Commun. 2011, 13, 570−573. (12) da Silva, L. F.; Avansi, W.; Moreira, M. L.; Mesquita, A.; Maia, L. J. Q.; Andres, J.; Longo, E.; Mastelaro, V. R. J. Nanomater. 2012, No. 890397. (13) Moreira, M. L.; Longo, V. M.; Avansi, W.; Ferrer, M. M.; Andres, J.; Mastelaro, V. R.; Varela, J. A.; Longo, E. J. Phys. Chem. C 2012, 116, 24792−24808. (14) Hu, L.; Wang, C.; Lee, S.; Winans, R. E.; Marks, L. D.; Poeppelmeier, K. R. Chem. Mater. 2013, 25, 378−384. (15) Toshima, T.; Ishikawa, H.; Tanda, S.; Akiyama, T. Cryst. Growth Des. 2008, 8, 2066−2069. (16) Zhang, J.; Bang, J. H.; Tang, C.; Kamat, P. V. ACS Nano 2009, 4, 387−395. (17) Xin, Y.; Jiang, J.; Huo, K.; Hu, T.; Chu, P. K. ACS Nano 2009, 3, 3228−3234. (18) Amani Hamedani, H.; Lee, S. W.; Al-Sammarraie, A.; Hesabi, Z. R.; Bhatti, A.; Alamgir, F. M.; Garmestani, H.; Khaleel, M. A. ACS Appl. Mater. Interfaces 2013, 5, 9026−9033. (19) Macak, J. M.; Tsuchiya, H.; Berger, S.; Bauer, S.; Fujimoto, S.; Schmuki, P. Chem. Phys. Lett. 2006, 428, 421−425. (20) Macak, J. M.; Tsuchiya, H.; Ghicov, A.; Yasuda, K.; Hahn, R.; Bauer, S.; Schmuki, P. Curr. Opin. Solid State Mater. Sci. 2007, 11, 3− 18. (21) Yang, J.; Geng, B.; Ye, Y.; Yu, X. CrystEngComm 2012, 14, 2959. (22) Kalyani, V.; Vasile, B. S.; Ianculescu, A.; Buscaglia, M. T.; Buscaglia, V.; Nanni, P. Cryst. Growth Des. 2012, 12, 4450−4456. (23) Scherrer, P. Nachr. Ges. Wiss. Göttingen, Math.-Phys. Kl. 1918, 26, 98−100. (24) Masahashi, N.; Semboshi, S.; Ohtsu, N.; Oku, M. Thin Solid Films 2008, 516, 7488−7496. (25) Hara, T.; Ishiguro, T. J. Ceram. Soc. Jpn. 2010, 118, 300−304. (26) Kuang, Q.; Yang, S. ACS Appl. Mater. Interfaces 2013, 5, 3683− 3690. (27) Setinc, T.; Spreitzer, M.; Vengust, D.; Jerman, I.; Suvorov, D. Ceram. Int. 2013, 39, 6727−6734. (28) Moon, J.; Kerchner, J. A.; Krarup, H.; Adair, J. H. J. Mater. Res. 1999, 14, 425−435. (29) Ye, L.; Mao, J.; Liu, J.; Jiang, Z.; Peng, T.; Zan, L. J. Mater. Chem. A 2013, 1, 10532−10537. (30) Wulff, G.; Kristallogr, Z. Mineral 1901, 34, 449−530. (31) Zheng, H.; Zhan, Q.; Zavaliche, F.; Sherburne, M.; Straub, F.; Cruz, M. P.; Chen, L.-Q.; Dahmen, U.; Ramesh, R. Nano Lett. 2006, 6, 1401−1407. (32) Sano, T.; Kim, C.-S.; Rohrer, G. S. J. Am. Ceram. Soc. 2005, 88, 993−996. (33) Murphy, C. J. Science 2002, 298, 2139−2141. (34) He, J. Q.; Jia, C. L.; Vaithyanathan, V.; Schlom, D. G.; Schubert, J.; Gerber, A.; Kohlstedt, H. H.; Wang, R. H. J. Appl. Phys. 2005, 97, 104921−1−104921−6. (35) Gilbert, S. R.; Wessels, B. W.; Studebaker, D. B.; Marks, T. J. Appl. Phys. Lett. 1995, 66, 3298−3300. (36) Zagar, K.; Recnik, A.; Ajayan, P. M.; Ceh, M. Nanotechnology 2010, 21, No. 375605.

I

dx.doi.org/10.1021/cg500374m | Cryst. Growth Des. XXXX, XXX, XXX−XXX